In an electric double layer capacitor, a polarizing electrode including activated carbon is fabricated by kneading the activated carbon together with an electrically conducting material and a bonding material. Another known electric double layer capacitor is fabricated by mixing activated carbon and uncarbonized phenol resin, sintering the mixture to form solid electrodes acting as positive and negative electrodes, respectively, placing these two electrodes on opposite sides of a separator, bringing electric charge collectors into contact with the electrodes and impregnating the electrodes with an aqueous or nonaqueous electrolyte. It is common practice to fabricate the positive and negative electrodes from the same amount of the same material.
Japanese Unexamined Patent Publication No. 203614/1986 describes an electric double layer capacitor having positive and negative electrodes fabricated from the same amount of the same material. These two electrodes are equal in electrostatic capacity. This Japanese Publication states that when the capacitor is charged and discharged with a voltage corresponding to the withstand voltage, deterioration of the internal resistance and the electrostatic capacity is observed. Deterioration of the characteristics due to high-voltage application is prevented by matching the electrostatic capacities of the positive and negative polarizing electrodes to the ratio of the inverses of the reaction potentials of the positive and negative electrodes, respectively, when the null potential of the potential-current characteristic curve is used as a reference.
Japanese Patent Publication No. 847/1990 describes a method of moderating similar problems. This is achieved by making different the amounts of positive and negative polarizing electrodes. However, careful observation of the charging and discharging operation of the capacitor having this structure reveals the following problem. When the power supply is turned on, i.e., in the initial stage of charging, the voltages that the positive and negative electrodes must bear, respectively, are distributed according to the inverse proportion to electrostatic capacity. As the charging and discharging are repeated, the voltages that the electrodes must bear are distributed gradually at the ratio of leakage resistances. Consequently, the original purpose cannot be achieved.